Close-Up--Plate Tectonics Theory

Midocean Ridges

The midocean ridges are rifts in the ocean floor through which very hot magma wells up from
the mantle. This creates new oceanic plates and these move away from the ridge. As new magma
wells up and intrudes into the seafloor, the older crust is moved out
away from the midocean ridge like a conveyor belt. As well, the heat from the mantle causes
the ridge to swell upward and this plus the outpouring of lava create an undersea mountain ridge that is higher
than the cooler rock on either side of it. That is, the cooled crust settles down. Hence, an immense
ridge system like a huge mountain belt, runs through all the oceans and makes a continuous
mountain range some 40,000 miles long.
When magma solidifies, it freezes and locks in a magnetic polarity corresponding
to that of the Earth's magnetic field. Over long periods of time, the
Earth's polarity changes (this is called a magnetic reversal); that is, the magnetic north pole becomes the
magnetic south pole, and so the polarity of the newly formed crust
changes as well. The conveyor beltlike action of plates moving away from the ridge produces
a series of
strips of rock magnetized in opposite senses, with the magnetic stripes
parallel and symmetrical to the ridges. On either side of the ridge you will have a similar magnetic
striping (and the rocks will be the same age), similar to two identical mirror image bar codes on either side of the ridge.